The present invention relates to a thermal flow sensor for measuring a flow and a flow velocity of fluid.
FIG. 3 is a diagram showing a construction of a conventional thermal flow sensor, for instance, disclosed by Unexamined Japanese Utility Model Application (OPI) No. Sho. 61-108930. A detecting tube 202 is provided at a predetermined position inside a housing 201 along a main flow passage of the fluid.
A flow velocity probe 203 provided with a heat-sensitive resistor 306 (as shown in FIG. 4) and a fluid temperature sensor 204 are disposed at a predetermined position inside the detecting tube 202. These two components form a bridge circuit with resistances R.sub.1 and R.sub.2. Nodes b and c of the bridge circuit connect to input terminals of a differential amplifier 101. An output of the differential amplifier 101 is connected to a base of a transistor 102, while an emitter of the transistor 102 is connected to an end a of the bridge circuit, and a collector of which is connected to a power source 103.
FIG. 4 is a plan view showing a flow velocity probe 203 of another conventional thermal flow sensor disclosed by Unexamined Japanese Patent Application (OPI) No. Hei. 2-269915, and FIG. 5 is a side view of the flow velocity probe 203. As shown in FIGS. 4 and 5, an insulation substrate 305 is provided, and a heat-sensitive resistor 306 is attached on the substrate 305, which is formed as a film and made of a material whose resistance varies with temperature. A patterning line 307 is drawn on the heat-sensitive resistor 306 for forming a current passage. Both lead wires 308 are connected to both ends of the heat-sensitive resistor 306.
FIG. 6 is a perspective view showing a detecting tube 402 and a flow velocity probe 403 of still other conventional thermal flow sensor so called as a heating wire type, disclosed by Unexamined Japanese Patent Application (OPI) No. Sho. 55-6294. The flow velocity probe 403 of this type does not have a support member. The flow velocity probe 403 is formed with only a wire material made of a heat-sensitive resistance material such as platinum wire in this prior art.
An operation of the conventional thermal flow sensor will be described as follows. When the fluid of a constant flow flows inside the housing 201, a current supplied to the bridge circuit is controlled by a control circuit constituted by the differential amplifier 101 and the transistor 102 in order that the temperature of the heat-sensitive resistor 306 provided in the flow velocity probe 203 may be higher than the temperature of the fluid by a predetermined temperature. Therefore, the bridge circuit is in a balanced state.
In this state, when the flow of the fluid increases, the resistance value of the heat-sensitive resistor 306 is changed by being cooled, so that the bridge circuit becomes in an imbalanced state, and then the current supplied to the bridge circuit is increased. Therefore, an average temperature of the heat-sensitive resistor 306 returns to the original temperature by heating. As a result, the balanced state of the bridge circuit is recovered.
During the above operation, the heat generated on the heat-sensitive resistor 306 is consumed for heating peripheral portions of the flow velocity probe 203 through the substrate 305 as well as heating the heat-sensitive resistor 306 itself. Since a ratio of the heat consumed for both purposes varies in accordance with the flow, a temperature distribution of the peripheral portions is also changed by the flow.
Although the average temperature is controlled in the balanced state, in the case where the flow of the fluid is rapidly changed, the control circuit acts transitively until the temperature of each of the peripheral portions reaches a temperature corresponding to the flow velocity, because the temperature of each of the peripheral portions varies in accordance with the flow velocity. Therefore, a correct output cannot be obtained from the control circuit during the transitive period.
An operation of still other conventional thermal flow sensor of the heating wire type is similar to that of the conventional thermal flow sensor described above. However, comparing the conventional thermal flow sensor with the sensor of the heating wire type, since the heating wire type flow sensor allows the flow velocity probe to be formed thinner, a conduction coefficient of the compulsive convection heat is improved. Further, the heat conductivity in the flow velocity probe is small and, accordingly, heat transmitted to the peripheral portions is also small.
However, since the flow velocity probe of the conventional thermal flow sensor is constituted as described above, a relatively long time is required until the temperature distributions of the flow velocity probe and peripheral portions of the flow velocity probe become in a balanced state when the flow of the fluid is rapidly changed. Therefore, there is a problem in terms of the responsiveness as a flow sensor. There is another problem in that unevenness of the heat capacity transmission from the substrate to the peripheral portions affects the characteristics of the flow velocity probe.
Moreover, with respect to still other conventional thermal flow sensors of the heating wire type, since an aspect ratio of the flow velocity probe is large, it is able to prevent the heat from transmitting from the flow velocity probe to the peripheral portions. However, a sufficient sectional area is required to obtain the necessary strength and to improve the durability of the flow sensor, and the length of the flow velocity probe is required to be longer. Therefore, there is another problem in that power consumption of the flow velocity probe undesirably increases.